1. Field of the Invention
The present invention relates to a display device including the input device for inputting coordinates to a screen, and more particularly, to a display device which includes an input device of a capacity coupling type and which is suitable for achieving high coordinate detection accuracy.
2. Description of the Related Art
A display device including an input device (hereinafter, also referred to as “touch sensor” or “touch panel”) having an on-screen input function of inputting information to a display screen by a touch operation (contact and press operation, hereinafter, simply referred to as “touch”) with a user's finger or the like is used for a mobile electronic device such as a PDA or a mobile terminal, various home electric appliances, a stationary customer guiding terminal such as an automatic reception machine, and the like. As the input device using the touch, there are known a resistance film method of detecting a change in resistance value of a touched portion, an electrostatic capacity coupling method of detecting a change in capacitance thereof, an optical sensor method of detecting a change in quantity of light at the portion shielded by the touch, and the like.
The electrostatic capacity coupling method has the following advantages when compared with the resistance film method or the optical sensor method. For example, the electrostatic capacity coupling method is advantageous in that, as opposed to a low transmittance of the resistance film method or the optical sensor method of about 80%, a transmittance of the electrostatic capacity coupling method is as high as about 90%, thereby preventing a reduction in displayed image quality. In the resistance film method, a touch position is detected by mechanical contact of the resistance film, thereby leading to possible deterioration or breakage of the resistance film. However, in the electrostatic capacity coupling method, there is no mechanical contact such as contact of a detection electrode with another electrode. Thus, the electrostatic capacity coupling method is also advantageous in durability.
An exemplary touch panel using the electrostatic capacity coupling method is discussed in JP 2003-511799 A. In the method disclosed therein, a vertical electrode (X electrode) and a horizontal electrode (Y electrode) are arranged for detection in vertical and horizontal two-dimensional matrix, and a capacity of each electrode is detected by an input processing unit. When a conductor such as a finger touches a surface of the touch panel, the capacity of each electrode increases. Thus, the input processing unit detects this capacity increase to calculate input coordinates based on a signal of a capacity change detected by each electrode. Even when the detection electrode is deteriorated to change a resistance value as physical characteristics, its influence on capacity detection is limited. Thus, there is only a little influence on input position detection accuracy of the touch panel. As a result, high input position detection accuracy may be realized.
However, in the touch panel using the electrostatic capacity coupling method, as discussed in JP 2003-511799 A, the capacity change of each detection electrode is detected, and input coordinates are detected. Hence, input means needs to be made of a conductive material. Thus, when a stylus made of a nonconductive resin used by the resistance film method is brought into contact with the touch panel using the electrostatic capacity coupling method, almost no capacity change occurs in the electrode, which disables detection of the input coordinates.
When a stylus is made of a conductive material such as a metal on the other hand, and an input operation is performed on the touch panel using the electrostatic capacity coupling method with the use of the stylus, the number of electrodes increases. For example, it is assumed that an electrostatic capacity coupling type touch panel having a diagonal size of 4 inches and a vertical and horizontal size ratio of 3:4 is realized in an electrode shape based on a diamond shape as described in JP 2003-511799 A. When a finger is an input target, if a minimum contact surface has a diameter of 6 mm, and detection electrodes are prepared by using this size as an electrode interval, the total number of electrodes is 22. If a contact surface of the stylus has a diameter of 1 mm, and detection electrodes are prepared by using this size as an electrode interval, on the other hand, the total number of electrodes is 139, which indicates an increase of about six times. The increased number of electrodes causes an increase in frame area necessary for laying wiring lines in the input processing unit, and an increase in number of signal connections with a control circuit, consequently lowering reliability against shock and the like. Further, a circuit area increases due to an increase in number of terminals of the input processing unit, thereby leading to a fear of a cost increase.
As apparent from the foregoing, in the electrostatic capacity coupling type touch panel as disclosed in JP 2003-511799A, a reduction in number of electrodes is a challenge when an input operation by using the nonconductive material and the input means having a small contact surface are dealt with.